The present invention relates to friction elements such as clutch facings and the method of making such elements using rovings of continuous glass fibers as both the reinforcing and friction material.
Friction elements, such as automotive clutch facings, are subjected to high rotational and high centrifugal forces which have been known to cause the friction elements to burst.
Major automotive clutch facing manufacturers require that the driven member of the clutch, with two facings attached, be heated to 500.degree. F. for 15 minutes and then be subjected to spin testing as a prerequisite to product acceptance. This hot burst strength is usually the most difficult test to be passed by a friction facing since at elevated temperatures the binder materials such as resin and/or rubber, lose strength rapidly thereby resulting in bursts at relatively low rotational speeds. Conventional clutch facings formed of asbestos yarn having an O.D. of 11 inches and an I.D. of 61/2 inches are known to withstand this test up to spin speeds on the order of between 8,000 and 9,000 rpm.
As spin strength requirements for clutch facings became more demanding, glass fibers were used to strengthen the facings while asbestos fibers were retained as the friction material. These endeavors were characterized by high unit costs and in some instances difficulty in keeping the composite structures flat, as for example where a woven glass fabric was applied to the rear surface of the annular asbestos friction element.
More recently it has been suggested to form a clutch facing from a plurality of bundles of parallel continuous glass fibers spirally or randomly wound upon themselves in the form of an annular disc impregnated with heat-curable cement, there being filament means helically encircling portions of the periphery of the glass fiber bundles. Such construction is described in U.S. Pat. Nos. 3,743,069 and 3,756,910. Although such construction is alleged to provide somewhat improved hot burst strengths, as compared to clutch facings formed in large part of asbestos fibers; nevertheless, in the automotive industry clutch facings having even higher hot burst strengths than those provided by clutch facings made in accordance with these patents have been sought.
Applicant's discovery is that friction elements, particularly clutch facings, capable of withstanding the hot burst test at rotational speeds in excess of 12,500 rpm and even as high as 14,500 rpm, while maintaining the same physical dimensions, can be formed from bundles of continuous glass fibers wound randomly upon themselves, provided that said fibers are impregnated with a pre-coat composition comprising rubber, resin and furnace black prior to application of a heat-curable cement system comprising rubber, resin and friction modifiers.
It is an object of this invention to provide a novel glass fiber clutch facings of the above-described type which can be produced relatively inexpensively, yet have substantially increased burst strength.